This invention relates to a novel apparatus for superplastic forming of metals and more particularly to a relatively safe, stand alone apparatus for superplastic forming.
Superplastic metals are known in the art as materials that provide the strength of conventional metals and the elongation and formability characteristics of conventional plastic materials. Within a limited range of temperatures, certain metals such as titanium and certain aluminum alloys exhibit low resistance to deformation and may be elongated with controlled thinning. This characteristic of such metals occurs at certain strain rates within particular temperature ranges for the metals and is known as superplasticity. Typically, pressurized gas provides the force required to deform the metals. Details concerning a particular method of superplastic forming are described in U.S. Pat. No. 4,181,000 to Hamilton.
One characteristic of prior art superplastic forming devices is that they require the use of relatively large presses. These presses are large in order to have the capability of containing gas pressures of a high magnitude within the apparatus and assuring proper sealing. Consequently, prior art forming apparatus presses are typically inordinately large and require a power supply capable of producing high pressures and a sophisticated hydraulic system. Thus, prior art systems require excessive energy to operate. Moreover, the requirement that a high power press be used adds considerably to the cost and complexity of superplastic forming apparatuses. An example of a prior art device incorporating a hydraulic piston and cylinder type press is illustrated in U.S. Pat. No. 3,529,458 to Butler. It must be noted that the greater and faster it is desired to deform and elongate the superplastic workpiece in the die chamber the greater the power capability required of the hydraulic piston and cylinder arrangement.
In general, superplastic forming devices require the use of rather large presses as well as other devices and structures. Indeed, since prior art metallic dies dissipate heat quickly, large insulators must be used to prevent the escape of heat from the apparatus which would otherwise detract from the heat available to heat the workpiece. This adds considerable weight and bulk to the apparatus necessitating the use of supporting structures. With the exception of U.S. Pat. No. 3,739,617 to Stejskal disclosing a stand alone vacuum forming device, prior art superplastic forming devices are not stand alone or self-contained. In this regard, it must be pointed out that the Stejskal device uses vacuum forming methods in contrast to high pressure forming methods desired for mass production superplastic forming.
Typically, prior art devices incorporate the use of metallic dies. Metallic dies are used because of their durability and because of their ability to withstand the high compressive forces used in superplastic forming. Examples of such devices are in U.S. Pat. No. 4,087,037 to Schier and U.S. Pat. No. 3,668,917 to Komapsu. However, other materials have also been used in prior art die structures. For example, U.S. Pat. No. 3,529,458 to Butler incorporates a die composed of concrete material. Moreover, U.S. Pat. No. 3,739,617 to Stejskal incorporates a die composed of ceramic material. However, it must be noted that in Stejskal's apparatus, pressures within the apparatus do not exceed atmospheric pressure; therefore, pressures on the ceramic die of Stejskal's apparatus will not exceed pressures on the order of 14.7 pounds per square inch. Thus, the ceramic die structure of Stejskal is not required to withstand high pressure and consequently is not well adapted to very fast superplastic forming processes as is the goal in modern mass production assemblies. Although metallic dies have the capability of withstanding high pressures, their hard surface makes the die difficult to manufacture to close tolerances, especially if the die surface must have ridges or other surface irregularities.
It is desirable that the workpiece be cooled while it is in place in the die cavity and prior to removal therefrom in order to prevent further undesired deformation or distortion of the workpiece while at superplastic temperatures. Some prior art superplastic forming apparatuses merely allow the workpiece to air cool after pressure is lowered and the unit is opened. However, this prior art approach has the disadvantage that it allows the workpiece to deform or distort if pressure is dropped quickly and the unit is opened soon after the superplastic forming operation has taken place. If, instead, the workpiece is allowed to cool while still under pressure within the die, this slow cooling can substantially extend the total time required to form the workpiece. Therefore, in an effort to hasten the superplastic forming process some prior art devices have injected the workpiece with a quenching or cooling fluid prior to removal therefrom and often prior to significant pressure drop. U.S. Pat. No. 3,529,458 to Butler and U.S. Pat. No. 4,299,111 to Fayal are examples of this prior art technique of cooling the workpiece prior to removal from the die. However, in contrast to applicant's invention these systems have not expedited the process of heating the workpiece. U.S. Pat. No. 4,299,111 to Fayal also uses high velocity ejection pins to push the workpiece out of the mold after it has been slightly cooled; the slight cooling eliminates any tight fit between the workpiece and the die cavity. The disadvantage of this prior art system is that it requires precise control and timing of ejection pressure and cooling of the workpiece.
Although prior art devices may incorporate structures providing high gas pressures to the superplastic forming cavity, there are typically no safety features used in conjunction with these high pressure structures. Thus, in the event of excess pressure buildup in the die cavity exceeding the ability of the apparatus to withstand such pressures, serious damage may result to the forming apparatus, proximal structures or people in the vicinity. Prior art devices have not addressed the safety problems innerent in superplastic forming systems.